The Azores archipelago is located in the North Atlantic Ocean at about 1500 km from the European continent and 3900 km from the North America east coast. It is formed by nine volcanic islands and a few islets dispersed along a 600 km NW-SE direction axis (Fig.1). Despite some uncertainties about the discovery date of the Azores, it is known that the archipelago was first settled by the Portuguese in the second quarter of the 15th century, in the early thirties. In nowadays the Azores have about two hundred and fifty thousand inhabitants and its economy is strongly dependent from the agriculture, fisheries and tourism.

Geological Setting

The islands rise from the so called Azores Plateau, a thick and irregular area of the oceanic crust roughly limited by the 2000 meters bathymetric curve (Needham & Francheteau 1974). This structure is believed to be related with the presence of a deep mantle plume in the zone where the American, Eurasian and Nubian lithospheric plates meet forming the Azores Triple Junction (e.g. Searle, 1980; Luís et al., 1994).

Figure 2 – Azores region main tectonic structures

The main tectonic features that dominate the Azores region are the Mid-Atlantic Ridge (MAR), which crosses the archipelago between the islands of Faial and Flores with a general N-S direction, and the Azores-Gibraltar Fracture Zone that constitutes the Eurasian-Nubian plate boundary and extends from the MAR to the region of Gibraltar. This boundary includes the Terceira Rift (TR) and the Gloria Fault (GF). The Terceira Rift is a complex structure trending NW-SE extending from the MAR to the Gloria Fault along a line defined by Graciosa, Terceira and São Miguel islands and by several submarine banks and basins, including, in a broad sense, the WNW-ESE fracture systems of Faial, Pico and São Jorge islands. The Gloria Fault is an E-W dextral strike-slip fault well distinct in the bathymetry, which extends from the vicinity of Santa Maria Island to the east (Fig.2). Impressive submarine and subaerial volcanic rift zones and central volcanoes extend along the MAR and the TR.

Figure 3 – Epicentre of the July 9th, 1998 Faial earthquake and aftershocks.

This complex geological and geodynamic setting of the Azores explains the significant seismic and volcanic activity, including eruptions and degassing processes, registered in the archipelago. Important landslides triggered either by earthquakes, volcanic eruptions, extreme meteorological conditions and/or coastal erosion processes frequently affect the islands, and tsunamis related with earthquakes and landslides were reported to have occurred in the past.

Destructive Earthquakes and Present-Day Seismicity

Since the settlement of the Azores more than 30 important earthquakes and countless seismic crises resulted in thousands of deaths and severe damages. The first reported destructive earthquake took place on the October 22nd, 1522, and reached intensity X (MM-56) on São Miguel Island (Silveira et al., 2003). Vila Franca do Campo, the capital of the Azores at that time, was completely destroyed and more than 5,000 people died, many of which buried by major debris flows associated with two main landslides triggered by the earthquake (Marques et al.,

Figure 4 – The July 9th, 1998, Faial earthquake caused the collapse of more than 80% of the houses in some parishes.

2009). The Azores strongest earthquake occurred on the July 9th, 1787, with epicentre close to São Jorge island north coast and caused over 1,000 deaths. The most recent devastating events were the January 1st, 1980, 7.2 magnitude earthquake (Hirn et al., 1980) which affected Terceira, São Jorge and Graciosa islands causing the dead of nearly 60 people, and the July 9th, 1998, 5.8 magnitude earthquake (Senos et al., 1999) that hit Faial and Pico islands (Fig.3) causing 8 casualities while 1,700 people were left homeless (Fig.4).

The instrumental data for the last thirty years clearly demonstrates that most of the seismic activity in the Azores region takes place along the TR and at the MAR (Fig.5). Both tectonic and volcanogenic seismic crises were frequent during this period.

Volcanic Eruptions, Unrest Episodes and Degassing Areas

The first noticed Azores historical eruption occurred at Furnas Volcano and its final stage coincided with the settlement of São Miguel Island, sometime between 1439 and 1443 (Queiroz et al., 1995; Guest et al., 1999). Since then at least 28 volcanic eruptions of different nature and magnitude caused several human victims, killed animals and buried infrastructures and productive soils.

Figure 5 – Seismic activity in the Azores region for the last 30 years.

The majority of the subaerial events was dominated by strombolian and hawaiian eruptive styles producing basaltic scoria and lava flows. Eruptions of this nature occurred at São Miguel (1563 and 1652), Terceira (1761), São Jorge (1580 and 1808), Pico (1562, 1718, 1720) and Faial (1672). More explosive eruptions took place at São Miguel Island central volcanoes involving magmatic and hydromagmatic phases (1563, 1630). In the 1630 Furnas Volcano subplinian hydromagmatic eruption approximately 100 people lost their lives due to pyroclastic surges (Cole et al., 1995). During this event important landslides changed the morphology of the nearby coastal zone while pumice and ash fall covered almost all the island reaching as far as Santa Maria island about 80 km to the south.

Many of the recorded historical eruptions in the Azores region were of submarine origin (Chaves, 1960). In all the Azoreans memory stays the 1957-58 volcanic eruption of Capelinhos Volcano, close to the western coast of Faial Island (Machado, 1959). The event had surtseyan characteristics during the first stages of activity and changed to strombolian and hawaiian styles when the accumulation of ashes around the vent prevented the entrance of seawater into the crater. A major consequence of this eruption was the immigration of more than 4,000 people to the United States (Coutinho et al. 2010).

Figure 6 – Serreta submarine eruption (photo taken in January, 1999).

On December 1998, another submarine volcanic eruption started 10 km NW offshore Terceira Island, being characterized by the production of the so-called basaltic lava balloons (Gaspar et al., 2003). This Azorean type of submarine volcanic activity (Fig.6) is believed to be related with very fluid and gas-rich basaltic magmas. The reinterpretation of several historical records of submarine volcanic events shows that this type of eruption is not so rare all over the world.

More recently, in 2005, a volcanic unrest episode occurred in São Miguel Island, in the Fogo-Congro volcanic system, involving magma rising to shallow reservoirs (Wallenstein et al., 2007). The phenomenon was characterized by an intense seismic activity (Fig.7) and ground deformation that caused surface ruptures and triggered more than 250 landslides in the central zone of the island (Marques et al., 2007).

Present-day volcanic activity is expressed by hydrothermal manifestations characterized by low temperature fumaroles, steaming grounds, soil diffuse degassing areas and thermal and cold CO2 springs. The spatial distribution, intensity and physicochemical characteristics of such phenomena are depending from the seismic and volcanic activity and can be influenced by the meteorological conditions. The presence of degassing phenomena in (1) inhabited areas, (2) visited lava structures and even (3) open-air depressed zones constitutes an important public health issue (Baxter et al., 1999; Viveiros et al., 2009). In 1992 two people died at Graciosa Island when visiting the Furna do Enxofre lava cave and more recently in São Miguel and Faial islands some people were evacuated from their homes due to the presence of high indoor CO2 concentration.

The Azores are a region particularly vulnerable to slope instability due to geological, geomorphologic and meteorological factors. The volcanic nature and morphology of the islands, namely the existence of steep slopes developed on incoherent materials, condition the occurrence of landslides, which are typically triggered by earthquakes, volcanic eruptions or extreme meteorological events.

The previously mentioned events of 1522 and 1630 typify landslides related with earthquakes and eruptions, respectively. However, the meteorological conditions are the most common cause of this hazard. One of the most recent events occurred on October 31st, 1997, and was triggered by an extreme rainfall episode. Nearly 1000 shallow slope movements, mainly translational slides and debris-flow, were generated and one of them caused 29 casualties at the Ribeira Quente village in São Miguel Island (Fig.8). During this event 114 residents were left homeless, 36 houses were destroyed, several roads were cut, water and energy supply systems suffered serious damages and areas of fertile land became covered by mud and debris (Marques, 2004).

In the last 500 years at least 12 tsunamis generated by earthquakes or landslides reached the Azores archipelago (Cabral, 2009). Recent studies revealed that the higher magnitude event was the teletsunami generated by the 1755 Lisbon earthquake, with epicentre in the North Atlantic, SW of Portugal. The tsunami attained the Azores with a maximum run-up of about 15 metres in Terceira Island causing the death of 6 people. Local earthquakes, like the ones of 1757 or 1980, generated small tsunamis that didn’t cause any victims.

Rather important in the Azores are the tsunamis caused by landslides. Flank volcano collapses and high magnitude sea cliff landslides are known to have occurred in pre-historical times in several islands and some historical reports allow reconstructing more recent events. The major reported landslide-triggered tsunami took place in 1847, when a rotational slide occurred in a sea cliff with more than 350 metres high at the NW coast of Flores Island. The entrance of material into the sea originated a wave that affected both Flores and Corvo islands, being responsible by the death of 10 people and more than 100 injured.

Figure 9 – CVARG seismic operative room.

Monitoring, Alert and Warning Systems

The concept of natural multi-hazards assessment and monitoring has an important expression in the Azores due to the coupled occurrence of the above mentioned geological hazards. Such evidence determined the scientific and technical multidisciplinary development of the Centre for Volcanology and Geological Risks Assessment (CVARG) of the Azores University along the past 15 years and justify its advisory role near the regional and local civil protection authorities trough the Centre for Information and Seismovolcanic Surveillance of the Azores (CIVISA) an organization created by the Azores University and the Azores Regional Government to assure the implementation, development and maintenance of a multi-parametric monitoring program for the Azores region based on geophysical, geodetic, geochemical and meteorological continuous real-time data acquisition networks and discrete data sampling campaigns.

Figure 10 – Volcano monitoring GNSS field survey.

The Azores seismological monitoring network is composed by nearly 40 short-period and broadband seismic stations. Portable seismic stations and seismic arrays are used to complement the permanent observation network and follow specific seismic and/or volcanic crises. Data collected in this domain provides (Fig.9): the analysis of tectonic and volcanogenic earthquakes and volcanic tremor; the study of seismic source dynamics and fluid transport mechanisms; the evaluation of maximum historical intensities maps and seismic site effects; the analysis of buildings vulnerability to seismic solicitations; and the definition of scenarios to model the impact of future events.

Geodetic studies are supported by regular field surveys (Fig.10) and permanent GNSS and total stations networks coupled with meteorological stations to correct atmospheric effects. These techniques allow crustal deformation studies concerning the identification, location and characterization of point source mechanisms and magmatic intrusions temporal and spatial evolution. At local scale, total station networks and meteorological data acquisition systems are used to monitor slope instability.

Figure 11 – Soil CO2 flux degassing measurements.

The fluid geochemistry network includes CO2 and H2S flux permanent stations and 222Rn activity coupled with meteorological sensors to study the influence of external factors in the degassing processes. Additionally, temperature and water level measurements are being performed in water drilling wells to identify local stresses due to crustal deformation phenomena and/or shallow intrusions. In the scope of specific public health protection, gases and water chemical analysis, soil degassing anomalies maps and measurements of the atmospheric air composition in houses are performed regularly (Fig.11).

As above mentioned meteorological sensors are used coupled with geodetic and gas flux monitoring stations. Nevertheless, a new hydro-meteorological network is being installed in all the islands in the scope of a monitoring programme to predict in time and space floods and landslides through the integration of meteorological data with geological, morphological and hydrological parameters (Fig.12).

Figure 12 – Meteorological station from the floods and landslides early warning system.

Data acquired by field monitoring stations are transmitted to the CVARG/CIVISA Emergency Operations Centre (COE) in São Miguel Island and stored in the AZORIS Geodatabase, which is the support for multi-hazard analysis, vulnerability assessment, crises scenarios and alert and warning systems (Gaspar et al., 2004). Critical information on the ongoing seismic and volcanic activity and on any precursory signs of volcanic unrest or geological slope instability determine the activation of the CVARG Crisis Cabinet and is transmitted to the regional and local civil protection authorities trough official communication channels.

The Azores archipelago is exposed to different geological and meteorological hazards already responsible for thousands of deaths and severe damages. In order to detect precursory signals and develop early warning systems a multi-parametric monitoring network is under operation. Geophysical, geodetic, geochemical and meteorological independent techniques provide data that are integrated in the scope of a continuous hazard and risk assessment programme to support civil protection regional and local authorities.

Catarina Silva is a PhD Student at the Azores University. Member of the Gas Geochemistry Group of the Centre for Volcanology and Geological Risks Assessment since November 2004. Expert in geochemical monitoring, diffuse degassing processes and geostatistical methods for geochemical modelling. Her work is mainly related with 222Rn diffuse degassing in volcanic areas and public health risk assessment.

Fátima Viveiros is a PhD Student at the Azores University. Is member of theGas Geochemistry Group of the Centre for Volcanology and Geological Risks Assessment since September 2001 and responsible for the soil CO2 permanent monitoring network installed in the Azores archipelago. Expert in geochemicalmonitoring, diffuse degassing processes and geostatistical methods for geochemical modelling.

Gabriela Queiroz is a senior researcher in Volcanology at the Azores University and has been involved in several European and National research projects. Is director of the Centre for Volcanology and Geological Risks Assessment since 2004 where is responsible for the Risks Assessment Unit. She is member of the Azores University General Council and was Director of the Geosciences Department from the Azores University (2001-2003).

João Luís Gaspar is Assistant Professor at the Centre for Volcanology and Geological Risks Assessment of the Azores University and CEO of the Centre for Information and Seismovolcanic Surveillance of the Azores to support the Civil Protection. Expert in physical volcanology, hazard assessment and crises management. Awarded with the Order of Merit commend by the President of the Republic of Portugal (1998), he received an honored mention from the National Meteorological Institute (1998) and the prize of GIS personality of the year from ESRI Portugal (2010). He was director of the Geosciences Department of the Azores University (1997-2001), director of the Centre of Volcanology and Geological Risks Assessment (1997-2004), regional director for Science and Technology (2004-2008) and regional director for Land Use Planning and Water Resources (2008-2010).

Nicolau Wallenstein is Assistant Professor at the Azores University and Head of the Geosciences Department since June 2009. Is member of the Centre for Volcanology and Geological Risks Assessment since 1997. Expert in physical volcanology and eruptive history studies, he was the coordinator of the volcanic seismology group (2002-2010). Presently is the coordinator of the Azores group for infrasound studies and is the local scientific responsible by the operation of the IS42 CTBTO international monitoring station.

Paulo Amaral has a PhD in Volcanology and is researcher at the Azores University. Member of the Landslide and Flash Flood Research Group of the Centre for Volcanology and Geological Risks since 2003. His main expertise lies in numerical modeling of hydrological processes coupled to slope stability analysis. His work is mainly related to the geotechnical and hydrological characterization of volcanic soils and with the monitoring of cinematic and hydrological behavior of landsides.

Rui Marques is a PhD student at the Azores University. Member of the Landslide and Flash Flood Research Group of the Centre for Volcanology and Geological Risks Assessment since 2002. His main expertise lies in geomorphology, namely on landslides monitoring and numerical modelling analyses, using both deterministic and probabilistic approaches. His work is manly related with landslide monitoring, landslide hazard assessment in GIS environment and landslides early-warning systems development.

Teresa Ferreira is Assistant Professor at the Centre for Volcanology and Geological Risks Assessment of the Azores University. Her research activities are directed to the study and monitoring of active volcanoes and other geological hazards. She was director of the Geosciences Department of the Azores University (2003-2005) and coordinator of the Volcanological and Seismological Observatory of the Azores University (2004-2008). Since 2008 is the President of the Board of Directors of the Centre for Information and Seismovolcanic Surveillance of the Azores to support the Civil Protection.

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